a sysctl machdep.tsc_freq problem

hi,
i found a strange case, i have tested the three machines(PIII 800 &
FreeBSD4.9-stable,
P4-1.6G & FreeBSD4.8-release, P4-2.0G & FreeBSD5.2.1).
I use "sysctl -a | grep machdep.tsc.freq" to get the CUP speed. But, I
failed on the P4-1.6G machine. Nothing appears.
Should I set some additional stuff on the P4-1.6G machine? I have do
nothing specailly for the other two machines, but it works.
Best Regards
Ouyang Kai